When digging holes for construction footings, in order to support a sufficient load, it is necessary to dig a hole of a large diameter, pour the footing with a suitable footing material, such as concrete, up to a certain level within the hole, and after the footing material has hardened or set, to back-fill the hole to form a smaller diameter shaft to the soil surface. Conventionally, this is accomplished by inserting and centering a tube of the desired smaller diameter into the hole, and back-filling the hole volume around the tube with earth or rocks or both.
Many types of post hole diggers are known. Auger-type diggers are provided with a screw that is advanced into the ground, such as the motorized auger shown by Ovens (U.S. Pat. No. 4,961,471). Alternatively, a boring implement can be secured to an end of a post for insertion into the ground. The post is rotated to advance the implement to the desired depth, aided by a stream of water directed into the borehole (Charland, et al., U.S. Pat. No. 4,986,373). The implement is left in the ground after the post has been positioned.
Manual post hole diggers are perhaps the most common type being sold today. The familiar “clamshell” digger consists of four main parts: two long pole-handles and two clamshell digging cups connected to the handles. The handles are moved together to separate the cups and apart to bring the cups together. Digging is accomplished by plunging the digger into the ground with the handles together, followed by spreading the handles apart to force the cups together to grip a quantity of soil. The digger is pulled out, handles still held apart, and the load is then dumped off to the side. The process is repeated until the hole is the desired depth.
However, with this type of digger a principal problem is that as the hole is dug deeper, the handles cannot be spread apart sufficiently far to grip the soil unless the diameter of the top of the hole is made progressively larger—otherwise, the handles will bind with the top edge of the hole. This type of digger requires time and effort for the removal of additional soil, and provides a less firm foundation for a post set into the hole when the loosened soil is used as fill. Likewise, another drawback is that this type of digging tool is limited to making constant diameter post holes.
U.S. Pat. No. 5,320,363 to Burnham discloses a post hole digger having a bracket plate, a first shaft assembly and a second shaft assembly, each shaft assembly with a first end and a second end. The first ends of the shaft assemblies are connected to the bracket plate. A pair of handles are pivotally attached to the first ends, and a pair of digging blades are pivotally attached to the second ends of the shaft assemblies. As the handles are rotated with respect to one another, the first shaft assembly moves axially with respect to the second shaft assembly to open and close the digging blades. The shaft assemblies are held in spaced relation to one another during operation of the digger so that the hole has an approximately constant diameter. A detent mechanism is provided for indicating when the digging blades are oriented approximately parallel to one another for insertion into the ground. Extension rods increase the operating length of the shafts to allow digging deeper holes.
U.S. Pat. No. 6,227,317 to Severns discloses a garden auger having an auger blade with a substantially rectangular body section for use with a drill motor. The preferred auger body includes toed-in wings for rapidly clearing earth from the created hole and cutters formed at one end of the auger blade.
U.S. Pat. No. 7,347,276 to Basek discloses a manual garden tool for tilling or breaking up surface soil, having an elongate shaft with an actuator rotatably mounted at its lower end, tine(s) mounted to the actuator, and a cam surface at the lower end of the shaft in contact with a surface of each tine such that, as the actuator is rotated between the first and second positions, the upper portion of each tine rotates with respect to the actuator and the lower portion of the tine moves between extended and contracted positions. The actuator and tines are locked against rotation when the tines are in either position, with the tines being locked against movement by being seated in troughs of a spilot plate. The actuator and tines can be unlocked and moved up the shaft to provide clearance past seat walls for the tines to move between the extended and contracted positions.
U.S. Pat. No. 7,673,698 to Walker discloses a gardening hand tool for digging holes which comprises a pair of shovel blades aligned parallel to each other and are pivotally attached to the bottom of a shaft member. A handle is attached to the upper end of a shaft member and is pivotally connected to the shovel blades.
It would be advantageous if a digging tool could drill or bore a hole having a larger diameter at the bottom thereof, and a smaller diameter shaft above the bottom portion which would not need to be back-filled in the conventional manner.